An AS interface as a bit-oriented field bus on the lowest level of the automatization pyramid is described in:                the R. Becker (Ed.) publication AS Interface, the Design Approach in Automation, Gelnhausen (2002):                    ASInternational Association, in particular Chapter 3.2,                        the Internet reference de.wikipedia.org,        DE 197 43 981,        the standard EN 50295,        the Web page http://www.as-interface.com/whatisasi.asp,        the presentation “The AS-Interface Innovation Step 3.0”of the AS International Association from Jun. 28, 2005 and        the presentation “AS-Interface” http://www.emg.ing.tu-bs.de/pdf/IKF/AS-i_SS04.pdf.        
It is optimized for the connection of simple sensors, actuators, and field devices that exchange only a few information bits with the control. Thus, only four information bits are able to be transmitted from the master to the slave and back again in a data message from the AS interface.
However, modern field devices, as used in particular in systems having decentralized control technology, often require numerous parameters that may be variably adjusted to optimize the function of the field devices.
Moreover, modern field devices may often collect diagnosis and service information that supports a quick removal of problems in the event of a malfunction. This includes electronically transmittable data not only about the product, such as manufacturer, model designation, production date, state of firmware, but also about the operation, such as operating hours, number of start/stop procedures, overload states, etc.
Other field devices, for operating and observing, for example, have a simple display on which measurement data are output and a keyboard field via which commands may be input to the control, for example. Here too, different byte-oriented and bit-oriented functions are logically assigned to each other, the first ones being used for the display and the last ones being used for the keys.
To accommodate this condition, specification version 3.0 introduces slave profiles, which are provided with both bit-oriented and byte-oriented data exchange mechanisms, see the cited publication AS Interface, the Design Approach in Automation, for example. The profile S-7.A.5 is mentioned by way of example in this instance. In this profile, two bits are used in each instance for a serial data exchange; thus, only one or two bits remain for the quick bit-oriented data exchange. This in turn is too little for many applications—e.g., in drive engineering.